In many applications, for example power management, signal conditioning and signal isolation, high performance inductors formed by coils are needed. Planar coils comprise one or more turns of conductive material which generally all lie in the same plane (e.g. in the form of a flat helix) or in a small number of parallel planes (e.g. in the form of a plurality of helixes arranged in a stack of substantially parallel planes). The turns are connected by leads called “taps” to the outside. An assembly comprising the turns of the coil, the taps, the substrate on which the coil is fabricated, and the magnetic core is called a coil assembly. Planar coils have the advantage of relative low height compared to axial coils, thereby providing relatively a low package height and an overall smaller device.
There is a continuous desire to develop even more effective and compact inductors comprising coil assemblies for DC-DC converters, transformers, electrical motors for use in, for example, space, industrial, medical and consumer applications.
Preferably, coil assemblies comprising planar coils are surface mountable to a printed circuit board (PCB) in order to enhance the manufacturability of the incorporation of these coil assemblies into systems comprising further electronic devices on a PCB. For an electronic device to be surface mountable, it needs to be provided with contact pads on a surface of the device. These contact pads can then be provided with solder bumps which then are contacted to contact areas on the PCB, or said contact pads can be contacted to solder bumps present on contact areas on the PCB.
Traditionally, coil assemblies comprising at least one planar coil are fabricated by depositing (for example by electroplating) a coil conducting material (for example copper (Cu)) on a semiconducting or dielectric substrate. Thereafter, the turn pattern is patterned in a resist, and the coil conducting material is etched, thereby forming a planar coil. A magnetic core consisting of a first magnetic core plate, typically made of soft ferrite, is provided on one face of the substrate and a second magnetic core plate, typically also made of soft ferrite, is mounted on the opposite face of the substrate. The second core plate is placed in contact with the first magnetic core plate by means of protrusions from the second magnetic core plate which protrusions extend to the lower plate through holes provided in the substrate. By this arrangement of the coil assembly, the magnetic field is confined by the magnetic core plates above and below the coil and by any protrusions outside the perimeter of the outermost turn and any protrusions positioned inside the innermost turn.
To further increase the confinement of the magnetic flux and thereby increase the inductance it would be desirable to also have the magnetic core arranged in-between the individual turns of the coil. WO2010001339A2 teaches how to obtain a higher inductance through special back- and front-shielding. Here a coil is provided on a silicon substrate. A soft magnetic metal material is deposited on the top of the coil and it extends in-between the individual turns of the coil. A soft magnetic metal material is also deposited on the reverse side of the silicon substrate. Via holes are etched in the substrate, and these via holes are filled with soft magnetic material, thereby forming vias which couple the soft magnetic metal materials on the respective sides to each other, thereby increasing the magnetic confinement further. The vias are not electrically contacted to the coil.
In the above application the proportion of the height of the turns of the coil relative to the height of the total coil assembly is relatively low, since the height of the total coil assembly includes the thickness of the non-magnetic silicon substrate which does not contribute to magnetic confinement and inductance. The contacting of the coil is not described—it is merely mentioned that taps contact the turns of the coil.
U.S. Pat. No. 6,831,543 teaches a planar coil assembly mountable on the surface of a printed board, which assembly is said to have a small power loss and large inductance. This is achieved by providing a surface mountable coil assembly comprising a upper ferrite magnetic film, a lower ferrite magnetic film and a planar coil interposed therebetween, in which an opening is formed in the upper ferrite magnetic film above the planar coil terminal portion and an external electrode (corresponding to tap and contact pad in the present application) conductive with the coil terminal portion through the opening is formed on the upper ferrite magnetic film. It is further taught that the external electrode is preferably formed by treating conductor paste composed of mainly one of Ni, Pd, Pt, Ag, Au or alloy powder containing these materials or solder paste composed of mainly Sn by heat treatment. It is also taught that contamination halfway in the process could deteriorate the conduction from the coil terminal portion to the external electrode with accompanying voltage drop and, in the worst case scenario, failure of the device. This could be mitigated preferably by performing a light etching with acid or a clean with organic solvent before providing the external electrode. After forming the external electrode, a metal cap is formed which contacts the external electrode. The thickness of the lower ferrite magnetic film, which film is deposited, is limited to 100 μm. For the next thicker film thickness of 150 μm investigated the film peels and thus this greater thickness is shown to be unsuitable for use in a planar coil assembly. The thickness mentioned for the upper ferrite magnetic film is 40 μm.
U.S. Pat. No. 6,060,976 teaches a plane transformer which has a primary plane coil and secondary planes coils formed from a conducting film with an insulating resin film on its periphery. The primary plane coil and the secondary planes coils are fitted in a fitting groove formed on an upper surface of a first substrate (corresponding to first magnetic core plate in present application) composed of a magnetic substance. Obviously, the thickness of the substrate is not limited by film peeling or similar. The fitting groove has an entrance portion and an exit portion that both run out in a side surface of the first substrate. The coils are obtained by punching a stack of plural types of resin films with incorporated copper foil into a shape similar to that of the fitting groove, which copper foil has a thickness of approximately several tenths of μm. This is followed by coating the stack with resin film by dipping such that the side surface of the stack is coated by resin, and then the stack is dried. The coils are then inserted and fitted into the fitting groove. End portions of the secondary plane coils and the primary plane coil are positioned in an entrance portion and an exit portion of the fitting groove. The end portions of the coils have the resin removed, and thereby conductors are exposed, to which leads are connected. U.S. Pat. No. 6,060,976 do not teach how the leads are connected or if this could be made as a surface mountable device. On the upper surface of the first substrate a second substrate (corresponding to the second magnetic core plate in present application) of magnetic substance is mounted, which second substrate has a gap insulating layer of a thickness preferably between 1 and 50 μm provided on the surface facing the first substrate.